REPRESENTING MOLECULAR SHAPE AND INTERACTIONS - A REDUCED INTERMOLECULAR POTENTIAL FOR COPPER PHTHALOCYANINE

The potential energy between molecules is often represented as a sum o f pairwise additive potentials for all atom pairs in both molecules. S uch atomistic potentials encode much physical and chemical information , and in particular give an accurate representation of the molecular s hape. However, the number of terms in the sum for a pair of molecules goes as N-2 where N is the number of atoms in a molecule, and thus gro ws rapidly inefficient for large N. Starting with an atomistic pairwis e additive potential for Copper Phthalocyanine (CuPc), a planar tile-s haped molecule with 57 atoms, we determine a simpler reduced intermole cular potential consisting of a sum of effective pair interactions bet ween 13 appropriately chosen ''interaction sites'' on each molecule. T his potential reproduces many qualitative features of the full atomist ic potential model for CuPc including the very anisotropic molecular s hape, but is much easier to evaluate numerically, requiring only 1% as much computation time as the full atomistic potential. Crystal struct ures of CuPc using both the atomistic and reduced potentials are deter mined and compared, and a discussion of diffusion barriers is given. S ome of the general issues and physical considerations that arise when attempting this reduction are discussed along with other possible appl ications of these ideas. (C) 1996 American Institute of Physics.